
package java_cup.runtime;

import java.util.Stack;

/**
 * This class implements a temporary or "virtual" parse stack that replaces the
 * top portion of the actual parse stack (the part that has been changed by some
 * set of operations) while maintaining its original contents. This data
 * structure is used when the parse needs to "parse ahead" to determine if a
 * given error recovery attempt will allow the parse to continue far enough to
 * consider it successful. Once success or failure of parse ahead is determined
 * the system then reverts to the original parse stack (which has not actually
 * been modified). Since parse ahead does not execute actions, only parse state
 * is maintained on the virtual stack, not full Symbol objects.
 *
 * @see lr_parser
 * @version last updated: 7/3/96
 * @author Frank Flannery
 */

@SuppressWarnings("unchecked" )
public class virtual_parse_stack {
    /*-----------------------------------------------------------*/
    /*--- Constructor(s) ----------------------------------------*/
    /*-----------------------------------------------------------*/

    /**
     * Constructor to build a virtual stack out of a real stack.
     */
    public virtual_parse_stack(Stack shadowing_stack) throws java.lang.Exception {
        /* sanity check */
        if (shadowing_stack == null)
            throw new Exception("Internal parser error: attempt to create null virtual stack");

        /* set up our internals */
        real_stack = shadowing_stack;
        vstack = new Stack();
        real_next = 0;

        /* get one element onto the virtual portion of the stack */
        get_from_real();
    }

    /*-----------------------------------------------------------*/
    /*--- (Access to) Instance Variables ------------------------*/
    /*-----------------------------------------------------------*/

    /**
     * The real stack that we shadow. This is accessed when we move off the bottom
     * of the virtual portion of the stack, but is always left unmodified.
     */
    protected Stack real_stack;

    /*
     * . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
     */

    /**
     * Top of stack indicator for where we leave off in the real stack. This is
     * measured from top of stack, so 0 would indicate that no elements have been
     * "moved" from the real to virtual stack.
     */
    protected int   real_next;

    /*
     * . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
     */

    /**
     * The virtual top portion of the stack. This stack contains Integer objects
     * with state numbers. This stack shadows the top portion of the real stack
     * within the area that has been modified (via operations on the virtual stack).
     * When this portion of the stack becomes empty we transfer elements from the
     * underlying stack onto this stack.
     */
    protected Stack vstack;

    /*-----------------------------------------------------------*/
    /*--- General Methods ---------------------------------------*/
    /*-----------------------------------------------------------*/

    /**
     * Transfer an element from the real to the virtual stack. This assumes that the
     * virtual stack is currently empty.
     */
    protected void get_from_real() {
        Symbol stack_sym;

        /* don't transfer if the real stack is empty */
        if (real_next >= real_stack.size())
            return;

        /* get a copy of the first Symbol we have not transfered */
        stack_sym = (Symbol) real_stack.elementAt(real_stack.size() - 1 - real_next);

        /* record the transfer */
        real_next++;

        /*
         * put the state number from the Symbol onto the virtual stack
         */
        vstack.push(new Integer(stack_sym.parse_state));
    }

    /*
     * . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
     */

    /** Indicate whether the stack is empty. */
    public boolean empty() {
        /*
         * if vstack is empty then we were unable to transfer onto it and the whole
         * thing is empty.
         */
        return vstack.empty();
    }

    /*
     * . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
     */

    /**
     * Return value on the top of the stack (without popping it).
     */
    public int top() throws java.lang.Exception {
        if (vstack.empty())
            throw new Exception("Internal parser error: top() called on empty virtual stack");

        return ((Integer) vstack.peek()).intValue();
    }

    /*
     * . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
     */

    /** Pop the stack. */
    public void pop() throws java.lang.Exception {
        if (vstack.empty())
            throw new Exception("Internal parser error: pop from empty virtual stack");

        /* pop it */
        vstack.pop();

        /*
         * if we are now empty transfer an element (if there is one)
         */
        if (vstack.empty())
            get_from_real();
    }

    /*
     * . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
     */

    /** Push a state number onto the stack. */
    public void push(int state_num) {
        vstack.push(new Integer(state_num));
    }

    /*-----------------------------------------------------------*/

}
